1. Field of the Invention
The invention relates to a control system for an automatic transmission. More specifically, it relates to a control system for reducing a shift shock during a down shift.
2. Description of Related Art
An automatic transmission performs gear shift by changing power transmission lines through planetary gear units in a gear train. Frictional engagement elements connected to a particular rotational element of the planetary gear units are properly engaged/disengaged by hydraulic servos so that the power transmission lines are changed. An electronic control system controls a hydraulic control system. The hydraulic control system controls the hydraulic servos.
In this automatic transmission, when the downshift from a high speed gear stage to a low speed gear stage is performed, a frictional engagement element engaged in the high speed gear stage is disengaged and a frictional engagement element engaged in the low speed gear stage is engaged. At that time, when the timing between the disengagement and the engagement is improper, an engine racing or a tie-up shock occur. Japanese patent application laying-open No. 63-266258 describes a feedback control to prevent engine racing and tie-up shock. In this control, when the aforementioned down shift is performed, an input rotation speed of the automatic transmission is changed smoothly at around the synchronizing rotation for the low speed gear stage, then it takes the rotation change of the rotational member into a target rotation change by the feedback control to prevent engine racing and tie-up shock.
In an automatic transmission, the frictional engagement elements are controlled through the hydraulic control system. Therefore, even when the electronic control system controls electrically and minutely, it is difficult to perform proper follow-up control because of a limitation of hydraulic responsiveness and a dispersion of mechanical character of the frictional engagement elements. Especially, in the aforementioned art, the rotation of the rotational member is controlled to achieve the target rotation change set based on the rotation in the shift transition. Therefore, a complicated control is needed.